Lithium-ion (Li-ion) has become the dominant rechargeable battery chemistry for consumer electronics devices such as smart phones and notebook computers and is poised to become commonplace for industrial, transportation, and power-storage applications. Li-ion battery chemistry is different from other rechargeable battery chemistries such as nickel metal hydride [NiMH], nickel cadmium [NiCad], and lead acid in a number of ways. From a technological standpoint, because of high energy density, Li-ion technology has enabled entire families of portable devices, such as smart phones. From a safety standpoint, a high energy density coupled with a flammable organic, rather than traditional aqueous electrolyte, has created a number of protection challenges. Specific challenges include the design of batteries containing Li-ion cells, and the storage and handling of these batteries.
An individual Li-ion cell will have a safe voltage range over which it can be cycled that will be determined by the specific cell chemistry. A safe voltage range will be a range in which the cell electrodes will not rapidly degrade due to lithium plating, copper dissolution, or other undesirable reactions. For most cells, charging significantly above 100% state of charge (SOC) can lead to rapid, exothermic degradation of the electrodes. Charging above the manufacturer's high voltage specification is referred to as overcharge. Since overcharging can lead to violent thermal runaway reactions, a number of overcharge protection devices are either designed into the cells or included in the electronics protection packages for Li-ion battery packs.
A Li-ion battery (or battery pack) is made from one or more individual cells packaged together with their associated protection electronics. By connecting cells in parallel, designers increase pack capacity. By connecting cells in series, pack voltage is increased.
Often, energetic failures lead to thermal runaway. Cell thermal runaway refers to rapid self-heating of a cell derived from the exothermic chemical reaction of the highly oxidizing positive electrode and the highly reducing negative electrode; it can occur with batteries of almost any chemistry.
If overheated or overcharged, Li-ion batteries may suffer thermal runaway and cell rupture. In extreme cases this can lead to combustion. Deep discharge may short-circuit the cell, in which case recharging would be unsafe. To reduce these risks, Lithium-ion battery packs contain fail-safe circuitry that shuts down the battery when its voltage is outside the safe range of 3-4.2 V per cell. When stored for long periods the small current draw of the protection circuitry itself may drain the battery below its shut down voltage; normal chargers are then ineffective. Many types of lithium-ion cell cannot be charged safely below 0° C.
A major component of a battery pack along is the battery management circuitry. Typically a battery management unit (BMU) consists of a charger and a fuel gauge (FIG. 1). The lithium ion battery pack itself consists of a lithium ion cell, a protection IC and protection FETs (charge (CHG) and discharge (DSG)). Protection circuits occupy useful space inside the cells, add additional points of failure and irreversibly disable the cell when activated. They are required because the anode produces heat during use, while the cathode may produce oxygen. These devices and improved electrode designs reduce/eliminate the risk of fire or explosion. Further, these features increase costs compared to nickel metal hydride batteries, which require only a hydrogen/oxygen recombination device (preventing damage due to mild overcharging) and a back-up pressure valve.
The function of the protection IC is to open the protection switches and detach the lithium ion battery pack from the system when the lithium ion cell voltage or current crosses critical safety thresholds. The various critical thresholds include over current during charge, over current during discharge, over voltage during charge, under voltage during discharge, short circuit during discharge, over temperature, and reverse current.
Some systems now use a non-removable pack. However, conventional pack-side protection is still typically used. This imposes some circuit redundancies and mechanical limitations. The protection FETs and the protection ICs add considerable cost to the battery pack. Furthermore, the PCB for the protection FET and ICs increases the weight, size, assembly time, and test time. There are heretofore unaddressed needs with these previous solutions.